Human productivity and quality of life has been substantially increased in recent years by the use of efficient climate control equipment for air conditioning and heating. The demands of food preservation and high technology instrumentation, equipment, and industrial processes have similarly required effective thermal control. Many of these needs have been effectively satisfied by the use of the reverse Rankine Cycle refrigeration system, the basic system of which consists of a refrigerant vapor compressor discharging into a condenser which liquefies the refrigerant while it rejects heat to the surroundings, followed by an expansion means which partially vaporizes the liquid refrigerant and lowering its temperature by virtue of absorbing its latent heat of vaporization. The refrigerant then passes to the evaporator which vaporizes the remaining liquid and absorbs heat from the cooling space, this vapor so produced is then returned to the compressor to repeat the cycle again.
As effective and commonly used as this system may be, it has three major drawbacks, all of which are associated with the use of the compressor:
The first is that the compressor is usually driven by high quality mechanical power provided by means such as an electric motor, mechanical power extraction from an engine, and the like. There are many applications where waste energy, either thermal or kinetic, is available and must be discarded and lost. One such example application is in automobile air conditioning where in the present state-of-the-art, a conventional reverse Rankine cycle refrigerator is used such that the compressor is powered directly through a belt drive by the engine, while large amounts of energy are lost through the engine exhaust and the cooling system. The energy extracted from the engine in this system to power the air conditioner clearly requires the expenditure of fuel with added cost and a resulting additional contribution to the pollution of the environment.
The second problem associated with current technology is that recent research has shown the chlorofluoromethanes (CFC's), the refrigerants of choice for the reverse Rankine cycle refrigeration system, have produced dire effects for the earth's ozone layer. Alternate refrigerants have been sought, particularly among the hydrochlorofluorocarbons (HCFC's), however, concerns have been raised about other environmental problems associated with these refrigerants. On the other hand, common water is an excellent refrigerant which has a high latent heat of vaporization, a high specific heat, good heat transfer characteristics, and is totally in harmony with the environment. However, for normal air conditioning applications, the specific volume of water vapor must be many times larger than that of a system using CFC's under comparable service requirements. Consequently, piston type compressors used with water vapor refrigeration systems require a much larger compressor displacement volume than those using CFC's as refrigerants. The need for a much larger compressor, and the associated increased cost, has rendered water to be less desirable as a refrigerant. However, this problem can be overcome by the use of an ejector which is capable of transporting large volumes of vapor within a relatively small space and at a low cost.
The third drawback of conventional reverse Rankine cycle refrigeration is the complexity of the compressor. This complexity increases the cost of the system, increases the maintenance required, and consumes excessive space or requires that the compressor be situated at inconvenient locations.
At the turn of the century when steam was more abundant than electricity, the patent literature reveals a plethora of inventions designed to produce refrigeration from thermal energy, thereby avoiding the aforementioned difficulties. These technologies were dominated by two classes of system: the ejector refrigeration system and the absorption cycle system.
In ejector refrigeration, a pump discharges condensate to a boiler/superheater which releases energetic primary vapor to the ejector. The ejector draws secondary vapor from the evaporator and discharges it to the condenser. The condensate from the condenser is divided between said pump inlet and the expansion means which supplies cool vapor/liquid to the evaporator. In accordance with the art of ejectors, the term "primary" is herein defined as the driving or energizing flow of the ejector. The term "secondary" is defined as the driven or energized flow. An early example of ejector refrigeration technology is disclosed in Hampson (U.S. Pat. No. 607,849) in 1898. Subsequent inventions disclosed strategies and methodologies for improving the performance of these systems by using various working fluids, multiple working fluids, and various heat sources (automotive waste heat, solar energy, geothermal power, combustion processes).
While absorption cycles received considerable attention, and still do, they have not competed well recently with conventional vapor-compression systems as a result of their higher weight, volume, and cost incurred by the need for the absorbent and the associated hardware for combining and separating absorbent and refrigerant. Although the system is capable of utilizing waste heat, the relatively low Coefficient of Performance (COP) of such systems has prevented adoption in many applications as well.
Ejector refrigeration has continued to draw considerable attention due is to its potential for low cost, its utilization of low-grade energy for refrigeration, simplicity, versatility in the type of refrigerant, and low maintenance due to the absence of moving parts. However, it has not, in fact, been widely adopted by because of the consequences of the low Coefficient of Performance that has hitherto been attainable. These consequences include higher energy consumption and the need for large and expensive condensers to handle the substantially higher thermal load.
It can be shown (e.g., Huang et al.,J. Engr. Gas Turb & Power,v107,1985) that the COP for a basic ejector refrigeration system operating at prescribed evaporator, condenser, and superheat conditions, is directly proportional to the ratio of the ejector secondary mass flow rate to the ejector primary mass flow rate. This ratio is commonly known to those skilled in the art of ejectors to be a fundamental figure of merit for the flow induction efficiency of an ejector. Hence, the higher the ratio of secondary to primary mass flow rates (mass flow ratio) the higher the COP. Thus, the performance of the ejector is a limiting factor in determining the performance of the refrigeration system. This has been recognized in the patent literature with many attempts at improving the ejector performance. This problem is discussed by Schlichtig (U.S. Pat. No. 3,199,310). Work (U.S. Pat. No. 2,301,839), for example, taught that a conventional ejector can produce a higher mass flow ratio if the molecular weight of the primary is much larger than that of the secondary. This led to the concept of a two fluid ejector refrigeration system which required a separator and two fluid circuits to handle each of the working fluids. While the thermal performance of the system was demonstrably improved, the additional volume and complexity of the system was a major deterrent for its adoption. Various improvements on the conventional ejector were proposed so as to increase its efficiency. Examples are given in Schlichtig (U.S. Pat. No. 3,199,310) for a multi fluid ejector design, Kemper (U.S. Pat. No. 3,277,660) who discloses a "novel multi phase ejector", Stein (U.S. Pat. No. 3,680,327) who discloses a "compound ejector" for use in steam-jet refrigeration which introduces multiple primary jets through stationary nozzles in order to enhance mixing, Modisette (U.S. Pat. No. 4,378,681) teaches the introduction of swirl into the ejector in order is to facilitate entrainment of fluid by virtue of an increased residence time of the primary fluid inside the ejector, Lauman (U.S. Pat. No. 4,748,826) teaches the enhancement of ejector mass flow ratio by suction through porous ejector walls, Seatinge (U.S. Pat. No. 4,905,481) replaces the conventional ejector with a supersonic venturi and teaches the advantages of various working fluids; and Kowalski (U.S. Pat. Nos. 5,117,648, 5,309,736) who discloses ejector designs in conjunction with special working fluids requiring sonic flow at the entrance in order to enable the ejector to function efficiently with saturated vapor at the inlet.
All of the attempts at improving ejector performance have involved variations on the conventional design, known is to those skilled in the art of ejectors as the "steady-flow" ejector. The physical principal upon which this device functions is that of entrainment of a secondary flow by an energetic primary flow by virtue of the work of turbulent shear stresses. Thus, the relatively low energy secondary flow is "dragged" by the relatively high energy primary flow through tangential shear stresses acting at the interface between the two contacting streams. These turbulent stresses are a result of mixing that occurs between primary and secondary streams and the consequent exchange of momentum. While this mechanism is quite effective, and has been widely adopted in many applications, an inherent characteristic of mixing processes is to dissipate valuable mechanical energy. This results in a substantial entropy rise, which is intimately connected with ejector performance, and consequently refrigeration system performance. This effect may be clearly seen in the example calculations displayed by FIG. 8 showing a dramatic decline in COP with ejector entropy rise. While a designer might wish to compensate for this loss of performance by increasing the primary mass flow rate, another commonly known limitation on ejectors is that the mass flow which can be transported through an ejector is limited by the phenomenon of "choking" at the section of minimum area in the ejector. Thus, as discussed by Huang, further increases in primary flow beyond the choked limit do not further increase flow induction proportionately and the COP of the system drops precipitously.
The optimal performance for ejector refrigeration could theoretically be achieved if there were no energy dissipation. An idealization of this standard may be modeled by replacing the ejector by ideal turbomachinery. Such a system is shown in FIG. 6. In such a system the primary flow from the boiler imparts energy to a turbine which expands the vapor into a condenser. The turbine transfers its energy by direct mechanical coupling to a compressor which takes suction of secondary flow from the evaporator and also discharges to the condenser. In the ideal situation, both turbine and compressor discharge at the same entropy and pressure to the condenser. If all processes are isentropic, this system will attain the highest COP possible for a basic ejector-refrigeration cycle for a given working fluid, evaporator temperature, condenser temperature, boiler pressure and superheat temperature. Such an invention was disclosed by Rice (U.S. Pat. No. 3,259,176). Although the ideal performance of this system is a standard for which ejector refrigeration systems can be compared, in practice, it suffers from some deficiencies which have limited its adoption. The principal problem is the high cost of the required turbomachinery. Furthermore, while turbomachinery is capable of providing extremely high efficiencies with large scale machines, at the small scales required of many common refrigeration applications, one would not expect that the efficiencies would approach ideal, thus there would be expected a significant entropy rise. This was acknowledged in Rice (U.S. Pat. No. 3,259,176). Hence, in comparison with a very low cost ejector, the expense of high quality turbomachinery may not be justified for most applications. Furthermore, the additional weight of turbomachinery, in comparison with an ejector, may be a consideration in a particular design.
The energy equation governing the exchange of energy between a primary fluid and a secondary fluid may be written: ##EQU1## where: h.degree.=specific stagnation enthalpy
t=time PA0 .rho.=density PA0 p=static pressure PA0 T=static temperature PA0 s=specific entropy PA0 u=fluid velocity PA0 f=force per unit volume due to turbulent and viscous shear stresses
If one considers a material element of fluid of infinitesimal volume and constant mass which enters the ejector through the secondary, the energy equation displays the physical mechanisms by which this secondary fluid element can acquire energy. Thus, the term on the left of Equation (1): ##EQU2## represents the acquisition of energy by the fluid element of unit mass as it moves through the ejector.
The first term on the right of Equation (1): ##EQU3## represents the reversible work of pressure forces upon the fluid element. This term exists only if the flow is non-steady in the laboratory frame of reference and is a result of the fact that stationary pressure forces can do no work. When the work of the non-steady pressure field is utilized to affect energy transport from an energetic primary fluid to a less energetic secondary fluid which are brought into direct physical contact with each other, the process is termed "pressure-exchange". Hence, this term relates to the physical mechanism upon which the present invention relies.
The second term on the right of Equation (1): ##EQU4## represents the energy received by the secondary as a result of heat transfer from the primary or from the environment and primarily contributes to the thermal energy level of the secondary rather than its mechanical energy.
The last term on the right of Equation (1): ##EQU5## represents the work of turbulent and viscous shear stresses and involves lateral mass and momentum transfer through irreversible transport processes. This is the process by which "steady-flow" ejectors rely. It is commonly known that this mechanism always results in a substantial entropy rise of the system.
The benefits of utilizing pressure exchange in a manner which employs compression and expansion waves are taught by Seippel (U.S. Pat. No. 2,399,394) who invented a class of pressure-exchanger often referred to as the "wave-rotor". This inventions comprise an assembly of cells rotating between fixed end walls having inlet and outlet openings for controlling the flow of the respective gas streams through the cells in succession. Advantage is taken of the compression and expansion waves which are set up in the succeeding cells by so locating the openings in the stationary end walls that each end of a cell is opened to an appropriate port substantially at the instant of the arrival at that cell end of a compression or an expansion wave. In these machines, one gas expands in such a manner as to compress another gas with which it is in direct contact. Means are provided for ducting to lead gas streams substantially steadily to and from the cells and means to effect rotation of the cells are provided.
The patent literature teaches several important properties of pressure exchanging devices. Seippel taught that if the flow passages were skewed in relation to the axis of rotation so as to affect an advantageous transfer of axial angular momentum, the rotors could be self-driven. Hertzberg (U.S. Pat. No. 3,367,563) taught that even with shock waves present, near isentropic energy transfer can be achieved. Hertzberg further taught that, contrary to the teaching of Work (U.S. Pat. No. 2,301,839), for a steady-flow ejector, the effectiveness of the pressure exchange process increases when the molecular weight of the driver gas is less than the driven gas as a result of the higher speed of sound in the lower molecular weight gas, enabling pressure waves to coalesce more effectively at the interface. The same effect occurs when the primary driver gas is of a higher static temperature than the driven secondary gas.
The benefits of pressure exchange and the deleterious effects of turbulent mixing on energy transfer are elucidated in Foa (U.S. Pat. No. 3,046,732). Foa also taught how pressure exchange could be effected in an ejector with interaction taking place externally to the rotor flow passages. This pressure exchange mechanism occurs as a result of the impact and deflection of the issuing jets of primary fluid from a plurality of nozzles, incorporated in a rotor, on the adjacent secondary fluid thereby creating a pressure field which rotates in accordance with the rotation of the rotor. Such a pressure field is non-steady in the laboratory frame of reference. It was further shown by Foa that if the orientations of the nozzles are skewed to the axis of rotation, the reaction of the issuing jet of primary fluid results in rotation of the rotor about its axis. Foa, however, contrary to the teachings of Seippel and Hertzberg, did not recommend the use of shock waves and expansion fans as a mechanism for creating the non-steady pressure field necessary for pressure exchange in flow induction devices.
The immense interest in pressure exchangers, wave rotors, and wave engines was largely concerned with propulsion applications and superchargers for internal combustion engines. Spalding (U.S. Pat. No. 3,140,928) taught that the "wave rotor" pressure exchanger concept could be used for "heat pump" applications whereby the pressure exchange process is utilized to produce "pressure equalization", the resulting equalized pressure being used for drying and distilling applications. No refrigeration function is disclosed. Other inventors have utilized the fact that when a gas expands isentropically, its temperature drops. By suitably arranging the ports of the wave rotor to collect expanded gas, cool gas can be extracted. However, nowhere in the prior art is it suggested to use pressure exchanging devices as part of a vapor-compression refrigeration system.
It is well known (e.g., Liepmann, H. W., Roshko, A.:"Elements of Gas Dynamics",pp124-130, John Wiley, 1957), that when a compressible fluid of given thermodynamic properties passes through a nozzle whose cross-sectional area initially decreases, then approaches a minimum area known as the "throat", and then increases to discharge the fluid at the nozzle exit, a pattern of compression and expansion waves may, or may not, appear at the nozzle exit depending on the ratio of the back pressure in the discharge region of the nozzle to the inlet stagnation pressure. If this pressure ratio is reduced below a certain level, a pattern of oblique shock waves and expansion waves appears at the exits of the nozzles. The various types of behavior, depending on pressure ratio, are indicated in FIG. 10.
There is an abundance of prior art on the utilization of engine waste heat for air conditioning purposes. Keller (U.S. Pat. No. 2,869,332) taught how steam generated by waste heat could be used to drive a turbine which would energize a conventional compressor in a vapor-compression refrigeration system. Ophir (U.S. Pat. No. 3,922,877) utilized waste heat of the engine of an automobile to power an ejector refrigeration system. Improvements on this theme were provided by Lowi (U.S. Pat. Nos. 4,164,850, 4,342,200) who discussed utilizing waste heat from the engine cooling jackets and from the engine exhaust. Presented in the preferred embodiments were two recuperators, viz., heat exchangers, to advantageously transfer heat from the warmer ejector vapor discharge to the cooler boiler/superheater feed condensate, thus reducing the amount of heat needed to be added in the boiler/superheater while reducing the amount of heat needed to be rejected in the condenser. The second recuperator disclosed is one which exchanges heat from the warmer liquid refrigerant before the expansion means and the cooler vapor discharging from the evaporator thus precooling the fluid to a temperature lower than that possible in the condenser. Huang also discussed the use of recuperators (using the altarnative terms "regenerator" and "precooler") and provided an analysis using experimental steady-flow ejector data showing how the COP of an ejector-refrigeration system is improved by the use of such recuperators. Ejector refrigeration improvements in automotive engines utilizing either cooling jacket waste heat or exhaust waste heat are disclosed in Briley (U.S. Pat. No. 4,523,437), Ohashi (U.S. Pat. No. 4,765,148), Fineblum U.S. Pat. No. (4,918,937), and Kowalski (U.S. Pat. Nos. 5,117,648, 5,239,837, 5,309,736).
It is well known that the ratio of the mass flow rate of secondary fluid to the mass flow rate of primary fluid diminishes as the quality of the primary vapor decreases because the ability of condensed droplets in a two-phase flow to entrain secondary vapor is minimal. For reasons previously explained, this has dire consequences for the COP of an ejector refrigeration system. Kowalski (U.S. Pat. No. 5,309,736) taught that condensation during the expansion of the primary vapor during mixing with the secondary in the ejector can be avoided by selecting a refrigerant having the property that its entropy when in a saturated vapor state decreases as pressure decreases. Although a variety of hydrocarbons were identified as meeting this requirement, many were toxic, flammable, or had otherwise undesirable characteristics as refrigerants.
An obvious method of avoiding condensation in the ejector is to employ a sufficiently superheated primary. This method was employed by Whitnah (U.S. Pat. No. 4,301,662). While this method is effective, ff the ejector is not efficient for other reasons, much of this additional heat added in the superheater must be rejected later in the condenser. The additional thermal loading on the condenser would tend to increase its size and weight. Hence, the benefit to ejector performance of superheating the vapor is mitigated by the additional energy supply requirement and the added heat rejection requirement. This dichotomy can be resolved if the improvement in ejector performance so reduces primary mass flow rate that both the total amount energy supplied and the amount of heat to be rejected in the condenser are diminished. Unfortunately, this has not always been the case and this is a reason why superheaters have not been universally adopted in ejector refrigeration systems.